Illicit drug use during pregnancy is a major risk factor for maternal and fetal morbidity. In addition, licit drugs are routinely administered to pregnant women, and therefore their fetuses, without the necessary data about the pharmacokinetics of these drugs in these vulnerable populations. The overall goal of this POI is to make use of drugs during pregnancy efficacious (licit drugs) and safer (licit and illicit drugs). We will achieve thi goal by testing the following overarching and unifying central hypothesis: Expression and activity of hepatic cytochrome P450 enzymes and placental drug transporters during pregnancy are regulated by pregnancy- related hormones and/or growth factors and by exposure to drugs and xenobiotics. Elucidating these mechanisms and quantifying the pregnancy-induced changes in these CYP and transporter activities will allow PBPK prediction of changes in maternal-fetal exposure to drugs througout pregnancy. The P450 enzymes and transporters we will study are those likely to be quantitatively most important for metabolism and transport of both illicit and licit drugs in the liver or the placenta, namely CYP3A (Project 1), CYP2B6 and CYP2D6 (Project 2), P-gp and BCRP (Project 3), and OCTS, NET, and SERT (Project 4). Pregnancy is known to induce expression and activity of hepatic CYP3A and CYP2D6, thus possibly lowering maternal exposure to drugs and potentially decreasing their efficacy. In the placenta, P-gp and BCRP participate in the efflux of drugs from the fetal compartment to the maternal circulation, thus protecting the fetus from adverse effects of drugs, while 0CT3, NET and/or SERT function to allow the entry of potentially toxic drugs into the fetal circulation. Thi POI uses a collaborative, synergistic, multidisciplinary and systems pharmacology approach to test the above-stated hypothesis. Results obtained from the proposed studies will allow us to predict how maternal and fetal exposure to licit and illicit drugs is affected by pregnancy and by gestational age and will reveal interesting and potentially novel mechanisms on physiological regulation of CYPs and transporters studied.